Method and installation for hot dip galvanizing hot rolled steel strip

ABSTRACT

A method for hot dip galvanizing of hot-rolled steel strip, wherein in a first method step, the strip ( 50 ) is introduced into a pickling station ( 10-13 ) and a layer of scale and reaction products are removed from the strip surface in the pickling station. In another method step, the strip ( 50 ) is introduced into a rinsing station ( 21-23 ) and residues of the pickle and pickling products are removed from the strip surface in the rinsing station, and subsequently the strip is introduced into a drying station and is dried. And from there, in another method step, the strip is introduced into a furnace ( 40 ) and is adjusted to galvanizing temperature under a protective gas atmosphere. In a last method step, the strip is guided through a galvanizing bath and the surface of the ( 50 ) is coated with a hot dip galvanizing layer in the galvanizing bath, wherein the strip temperature in the furnace ( 40 ) is adjusted at most to 50° K. above immersion temperature of the strip ( 50 ) into the zinc bath.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a method and an installation for hot dipgalvanizing of hot-rolled steel strip, wherein, in a first method step,the strip is introduced into a pickling station and a layer of scale andreaction products are removed from the strip surface in the picklingstation. In a second method step, the strip is introduced into a rinsingstation and residues of the pickle and pickling products are rinsed fromthe strip surface in the rinsing station, and subsequently, the strip isintroduced into a drying station and is dried in the drying station.From the drying station, the strip is introduced into a furnace inanother method step and is adjusted to galvanizing temperature in thefurnace under a protective gas atmosphere. In a last method step, thestrip is guided through a galvanizing bath and the surface of the stripis coated in the galvanizing bath with a hot dip galvanizing layer.

2. Description of the Related Art

Hot dip coating, particularly hot dip galvanizing, of hot-rolled steelstrip, so called hot strip, is becoming economically increasingly moreimportant as compared to conventional cold strip hot dip galvanizing. Asa result of the development of thin slab technology in hot-rolled strip,there is the technical possibility of producing hot strips in thethickness range of below 1.2 mm from casting heat. There is theadditional possibility to substitute cold strip for comparativelyinexpensive hot strip in dependence on the requirements of the customer.

Different methods and installations for hot dip coating, particularlyhot dip galvanizing, of steel strip are known. They are predominantlytypes of installation in which cold-rolled strips are used.

In such installations, the actual coating process is preceded by anannealing furnace in which a structural transformation takes place athigh temperatures for obtaining the desired mechanical properties. Theexisting temperature difference between the melt bath, preferably zincor zinc alloys, and the maximum strip temperature may be up to 400° C.However, hot dip galvanizing cannot be carried out with this overheatedstrip, so that the strip must be cooled prior to coating to temperaturesclose to the melt bath temperature.

In contrast, hot strip or preheated cold strip do not require annealingfor influencing the mechanical properties; rather, the strip temperatureis merely adjusted to that of the melt bath in order to achieve thedesired reaction of the steel strip surface with the alloying componentsof the melt bath. In contrast, high temperature annealing is frequentlydisadvantageous for the mechanical properties of the strip.

The present invention relates exemplary exclusively to the variousmethods of hot strip hot dip refining or hot strip hot dip galvanizing.

The desired temperature level, particularly for hot strip hot dipgalvanizing, is in the previously operated installations for hot dipgalvanizing still always higher than the required 450° C. of the zincbath. The reason for this is the required removal of all oxidationproducts and their prior stages from the steel strip surface. Oxidationproducts are inevitably produced in the transition area from thepickling stage through the rinsing and drying stage into the furnaceentrance due to the influence of ambient oxygen. The quantity andformation of the oxidation products entering the furnace and the ambientoxygen entrained by the strip determine the necessary method parametersof the treatment procedure, characterized by a required reductionpotential, temperature level and holding time. The temperature levelwhich is used is frequently so high that the strip must be additionallycooled prior to entering the zinc bath.

Another method of operation is characterized by a significant increaseof the temperature level in the zinc bath to values above 460° C. Aparticular disadvantage of this type of method is the increasedproduction of zinc-containing slag. On the one hand, this leads toincreased material and operating costs for the zinc bath and, on theother hand, to a reduced quality of the strip.

SUMMARY OF THE INVENTION

Starting from the prior art mentioned above, the invention is based onthe object of providing a method and a hot strip hot dip galvanizinginstallation which overcome the disadvantages and difficulties discussedabove and produce hot dip galvanized steel strip having a high anddefect-free surface quality with an economical amount of material andoperating costs.

For meeting this object, the invention proposes to adjust the striptemperature in the furnace at most to 50° K. above immersion temperaturein the zinc bath.

The H₂ concentration in the furnace is advantageously adjusted to atmost 20% and preferably to less than 5%. It is useful to carry out themethod steps between the last rinsing stage of the rinsing stationthrough the drying station up to the entrance of the heating furnacehermetically screened from the ambient oxygen from the surroundings.

Consequently, an installation for carrying out the method according tothe invention provides that the outlet of the last rinsing stage of therinsing station is connected to the inlet of the drier and the outlet ofthe drier is connected to the inlet of the furnace by locks and arehermetically sealed from the ambient atmosphere.

Additional useful further developments of the method and of the hot dipgalvanizing installation for hot strip are provided in accordance withthe features of subclaims.

The method and the installation according to the inventionadvantageously ensure that the optimum surface condition of the stripachieved after passing the strip through the pickling station and therinsing station is preserved in the subsequent drying stage as well asduring the transition in the furnace areas and from the furnace into thegalvanizing bath.

This is achieved by:

the above-mentioned adjustment of the temperature of the strip in thefurnace,

direct coupling of at least the last rinsing stage of the rinsingstation through the drying stage with the furnace inlet while screeningambient oxygen,

application of a water-binding medium, preferably NH₃, or a solutionthereof, onto the strip in the rinsing stage, wherein subsequently inthe drying stage the water-binding medium can be removed from the stripquickly and without residue, i.e., without the introduction of oxygen orliquid cleaning medium,

alternatively by operating the drying stage with an atmosphere which hasa reducing effect, for example, a N₂ H₂ gas mixture.

As a result of the measures mentioned above, the optimum strip conditionis preserved after pickling up into the furnace and an optimumadjustment of the strip temperature when it is immersed into the zincbath is achieved. The entrance of oxygen and the attendant surfacereactions, particularly oxidation, are prevented. This makes it possibleto operate the furnace at temperatures in the range of the melt bathtemperature. An overheating of the strip and a prolongation of theholding time in the furnace do not take place. A strip cooler is notnecessary. The manner of operation according to the invention and thecorresponding installation generally make possible a substantially morecompact construction of the furnace element and lower investment andoperating costs. Simultaneously, it is possible to operate the furnacewith low H₂ contents in the protective gas. The disadvantages of theconventional methods mentioned above with increased zinc bathtemperature are advantageously eliminated.

This is because, in accordance with the invention, the strip is adjustedto a temperature which is at most 50° K. higher than the immersiontemperature in the zinc bath.

Additional details, features and advantages of the invention result fromthe following explanation of an embodiment which is schematicallyillustrated in the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a layout of a hot dip galvanizing installation according tothe prior art,

FIG. 2 shows a layout of a hot dip galvanizing installation according tothe invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In accordance with the layout of a conventional hot dip galvanizinginstallation shown in FIG. 1, a strip 50 is introduced in a first methodstep into a pickling station 10 with three pickling stages 11 to 13 anda layer of scale as well as reaction products are removed from the stripsurface in the pickling station. Pickling is usually carried out in thepickling station 10 or in the pickling stages 11, 12, 13 by means ofhydrochloric acid (HCl).

In the subsequent method step, the strip 50 is introduced into therinsing station 20 with the rinsing stages 21 to 23 and residues of thepickle and pickling products are removed in the rinsing station from thestrip surface. Subsequently, the strip is introduced into and dried inthe drying station 30. From the drying station, the strip 50 isintroduced in another method step into a furnace 40 which comprises apreheating stage 41 and an integrated heating stage 42 and the strip isheated in the furnace to galvanizing temperature preferably under aprotective gas atmosphere. In a last method step, the strip is guidedthrough a galvanizing bath. In the galvanizing bath, the surface of thestrip 50 is coated with a hot dip galvanizing layer. In contrast to theconventional galvanizing installation according to FIG. 1, in accordancewith the layout according to the invention of the hot dip galvanizinginstallation according to FIG. 2, the method steps between the lastrinsing stage 23 of the rinsing station 20 through the drying station 30up to the inlet 43 of the heating furnace 40 are carried out while beinghermetically screened from the ambient oxygen from the surroundings.

By expanding the rinsing station 20 by a rinsing stage 23 or byscreening the rinsing stage 23 by means of a separating wall 24 from thepreceding rinsing stations 21, 22, a water-repellent or water-bindingmedium 25 is introduced into the rinsing stage 23. The medium used maybe, for example, NH₃ or a solution of NH₃.

A preferred development of the method provides that rinsing of the strip50 in the rinsing station 20 is carried out in the first stages 21 and22 with deionized water and in the third stage 23 with the addition ofNH₃ as a drying medium.

Drying of the strip 50 in the drying station 30 takes place without thesupply of air. In accordance with the invention, drying is carried outby means of thermal radiation with the addition of a mixture ofnitrogen, hydrogen and ammonia gas (N₂/NH₃) or H₂.

The drying station 30 is hermetically closed off against the entrance ofambient oxygen on both sides by means of locks 70, 80 adjacent thestations 20 and 40. The outlet of the last rinsing stage 23 of therinsing station 20 is connected to the inlet of the drying station 30and the outlet of the drying station 30 is connected to the inlet 43 ofthe heating furnace 40 through locks 70, 80, and they are hermeticallysealed from the ambient atmosphere.

The measures according to the invention maintain the optimum stripcondition after pickling up to the heating furnace because theintroduction of ambient oxygen is prevented. Consequently, as can beseen in the illustration of the heating furnace 40 in FIG. 2, theconstruction of the furnace can be simplified and realized with lowerinvestment and operating costs because of the lower necessary heatingpower and the omission of the cooling stretch. In addition, the furnaceoperation is possible with comparatively low H₂ contents in theprotective gas.

What is claimed is:
 1. Method for hot dip galvanizing of hot-rolledsteel strip, wherein: in a first method step, the strip (50) isintroduced into a pickling station (10-13) and a layer of scale andreaction products are removed from the strip surface in the picklingstation, in another method step, the strip (50) is introduced into arinsing station (21-23) and residues of the pickle and pickling productsare removed from the strip surface in the rinsing station, andsubsequently the strip is introduced into a drying station and is dried,and from there in another method step, the strip is introduced into afurnace (40) and is adjusted to galvanizing temperature under aprotective gas atmosphere, and in a last method step, the strip isguided through a galvanizing bath and the surface of the strip (50) iscoated with a hot dip galvanizing layer in the galvanizing bath, whereinthe strip temperature in the furnace (40) is adjusted at most to 50° K.above immersion temperature of the strip (50) into the zinc bath,wherein a water-repellent or water-binding medium (25) which wets thestrip (50) is introduced into the last rinsing stage (23) of the rinsingstation (20).
 2. Method according to claim 1, wherein the medium (25)introduced into the third rinsing stage (23) is NH₃ or a solutioncontaining NH₃.
 3. Method according to claim 1, wherein drying of thestrip (50) in the drying station (30) is carried out without the supplyof air from the outside by means of heat radiation with the addition ofa mixture of nitrogen, hydrogen and ammonia gas (N₂/NH₃)+H₂ or anothermixture of two of the mentioned gases.